Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”), which comprises an ion exchange membrane or solid polymer electrolyte disposed between two fluid diffusion layers (FDL). The FDLs can be formed from a variety of electrically conducting materials, such as carbon cloth, carbon paper, metal mesh, or expanded metal foil. The MEA contains a layer of catalyst, typically in the form of finely comminuted precious metal(s) at each membrane/FDL interface to form the electrodes that induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous FDL material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
Fuel cells have been identified as being particularly well-suited for numerous end-uses that employ electrical energy—from microelectronics to power plants. Fuel cells offer numerous advantages to conventional energy sources. For instance, unlike combustion-based energy sources, fuel cells emit virtually no harmful pollutants as reaction by-products. Fuel cells can also operate with renewable fuels, and they can reduce the need for power grid infrastructure since fuel cells can be located at the sites where electrical power is needed.
Of the many different fuel cell types, proton exchange membrane (PEM) fuel cells are thought to be best suited to low-power applications such as hand-held devices. PEM fuel cells can operate at lower temperatures (that is, below the boiling point of water), while still generating relatively high levels of electric energy. PEM fuel cell designs typically employ a polymeric ion exchange membrane as an electrolyte disposed between the cell's anode and cathode. Such ion exchange membrane have asymmetric properties. In this regard, while the membrane can effectively conduct positively charged protons, it blocks the flow of negatively charged electrons. The separation of the electrons and protons results in a voltage potential between the anode and cathode, such that current can flow through an electrical load when attached in circuit with the fuel cell. One of the most promising membrane electrolytes is Nafion®, which is a perfluorosulfonic acid based polymer commercially available from made by E.I. DuPont de Nemours & Company (DuPont).
Two or more fuel cells can be connected together electrically, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements in which neighboring fuel cells share oppositely facing surfaces of the same fluid flow field plate, current collection plate or other common component, such a series-connected, multiple fuel cell arrangement is generally referred to as a fuel cell stack, and is usually held together by tie rods and end plates. A plurality of individual fuel cells that are connected in series, but do not necessarily share a common component, is generally referred to as a fuel cell array. Fuel cell stacks and/or arrays typically include intake manifolds and inlet ports for directing the fuel stream and the oxidant stream to the anode and cathode, respectively. Stacks and/or arrays also generally include exhaust manifolds and outlet ports for expelling the exhaust fuel and oxidant streams.
One type of PEM fuel cell (a hydrogen/oxygen fuel cell) employs hydrogen gas as its fuel source. Although the use of hydrogen gas has many favorable properties for fuel cells (such as, for example, the production of pure water as the only reaction product), hydrogen gas is also the least dense of all possible fuels, and is also combustible. Because of its low density and combustibility, hydrogen gas is difficult and expensive to store and transport. Certain fuel cell systems, therefore, include fuel processing subsystems, in which a hydrogen feed stream is generated from a more readily storable and transportable organic feed stock, such as, for example, methane or methanol. In typical fuel processing subsystems, an organic feed stock is converted in a catalytic reactor (commonly referred to as a reformer) to a reformate stream that includes hydrogen and oxidized carbon compounds, including carbon dioxide and, in lesser amounts, carbon monoxide. Since the presence of more than trace amounts of carbon monoxide in the fuel cell feed stream can poison the fuel cell catalyst, selective oxidizer reactors are typically interposed between the reformer and the fuel cell array to further oxidize carbon monoxide to carbon dioxide. Although in significantly accumulated quantities CO2 can be detrimental to the earth's climate, it is benign in relation to fuel cell components when included in a reformate stream fed to a fuel cell. The use of organic fuels as the feed stock for hydrogen/oxygen fuel cells thus increases the size, weight, complexity and cost of hydrogen/oxygen fuel cell systems.
Another type of PEM fuel cell employs organic liquid fuels such as methanol, other alcohols, and organic acids including formic acid. The simplest of these organic liquid feed PEM cells directly oxidize an organic fuel stream without a need to create and store hydrogen gas. These direct organic liquid feed PEM fuel cells avoid problems associated with the use of hydrogen gas as the fuel stream. In direct organic liquid feed fuel cells, the organic fuel oxidizes at the anode, producing free electrons, hydrogen ions (protons) and carbon dioxide. The protons are conducted through the membrane electrolyte to the cathode. The electrons produced from the catalytic reaction at the anode are conducted through an external electrical circuit to the cathode. This migration of hydrogen ions across the membrane creates a voltage potential, and electrons flow from the anode through the electrical load to the cathode via the external circuit. In addition to conducting hydrogen ions, the membrane electrolyte isolates the liquid fuel stream from the oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
For many end uses, direct organic liquid fuel cells can produce sufficient electrical power using smaller, lighter, less expensive and fewer components than hydrogen/oxygen fuel cells. These qualities make them well suited for providing electrical power to low-wattage consumer electronic devices, such as cellular telephones, cameras, laptop computers, and personal digital assistants, as well as microsensors for analytical and strategic end uses. At present, however, organic liquid feed fuel cells are each capable of generating a relatively low voltage—generally less than 1 Volt apiece—making it necessary or desirable to assemble a plurality of fuel cells into a stack or an array to provide sufficient voltages to drive the desired electronic devices. The presence of multiple cells in an electric power generation system puts increased constraints on fuel cell developers to reduce size, weight and cost of individual fuel cells.
Most direct organic liquid feed fuel cell development to date has focused on using methanol as a fuel source. Methanol is relatively inexpensive, renewable and less flammable than hydrogen gas. However, to date, direct methanol fuel cell (DMFC) developers have been unable to demonstrate a low cost cell that operates efficiently. One shortcoming of DMFCs is that methanol fuel (as well as other alcohol fuels) interacts chemically with perfluorosulfonic membranes, including Nafion membranes. This interaction can result in methanol crossover or diffusion of methanol across the membrane from the anode to the cathode. Once the fuel has crossed the membrane and encountered the cathode, it can no longer participate in the anodic reaction, and moreover, the presence of methanol at the cathode inhibits the desired catalytic reactions at the cathode. In addition, the methanol that has crossed over to the cathode should be removed from the volume around the cathode.
To date, solutions for overcoming the problem of methanol crossover are prohibitively expensive. One such solution, for instance, requires expensive platinum-based catalysts to improve efficiency of the oxidation reaction. Other cost-prohibitive solutions include the use of pumps, sensors, filters and water management systems.
Because of the problems inherent in DMFCs, a few leading-edge fuel cell developers are shifting their focus to cells utilizing a different liquid organic fuel—formic acid. Formic acid, like methanol, is relatively inexpensive, renewable and relatively non-flammable (compared to hydrogen). However, unlike methanol, formic acid does not exhibit problematic levels of crossover caused by interactions with the ion exchange moieties present in perfluorosulfonic acid membranes, such as, for example, those formed from Nafion. The absence of fuel crossover reduces the need for expensive catalysts and other system components associated with methanol-based fuel cell technologies. Because of these lower-cost advantages, direct formic acid fuel cells (DFAFC) show significant promise as a replacement for conventional battery technology in low-power consumer electronic devices.
The use of formic acid in consumer electronics presents some unique concerns in the development commercially viable DFAFCs. First and foremost, formic acid is itself an acid, and can cause skin and eye irritations. Formic acid is also capable of reacting with electrical components, plastics and textiles. DFAFCs employed as power sources low-power consumer electronic devices should therefore prevent formic acid leakage from within the fuel cell structure. At the same time, a DFAFCs should employ a convenient mechanism for venting the anode and refueling the cell. A suitable fuel cell sealing solution that accommodates refueling and venting would promote the commercially viability of DFAFCs.
As with DMFCs, the formic acid fuel should be contained within the anode to promote efficiency and reduce costs. As previously discussed in connection with DMFCs, once the fuel migrates across the membrane to the cathode portion of the fuel cell, a host of problems arise, including loss of fuel for the anodic reaction, contamination of the cathode, and the need to remove fuel from the cathode. A suitable containment technique would inhibit or prevent formic acid leakage into the volume around the cathode or into the external volume surrounding the fuel cell itself.
In attempting to overcome the problem of formic acid leakage, previous DFAFCs have employed techniques to effect sealing while and the same time reducing the size, weight and cost of the DFAFC. Some prior techniques include the use of bulky and expensive hardware such as screws, bolts, and fasteners. Other techniques require complicated pressing techniques at high temperatures under highly controlled conditions. Yet another drawback of previously available assemblies is that the designs impose compressive forces on the sealing components which can cause these components to physically deteriorate and sometimes tear.
A small, light and inexpensive DFAFC is needed to effective seal formic acid within the fuel cell and that can be assembled without costly production techniques. Such a device should where possible reduce physical stresses imposed on the sealant layers of DFAFCs. Preferably, the DFAFC design should also accommodate the need to make a stack or array of cells. Finally, a commercially viable DFAFC design should allow for simple refueling of the device and for adequate ventilation of the anodic reaction.